It needs Thief, obviously, for the dagger specialization. However, Thief has few actual combat powers and is comparatively heavy on non-combat utility abilites (lockpick, traps, find treasure).

But now I'm trying to cover both some defense and a way to deal with crowds of monsters.

Weaponmaster, Gladiator and Reaver have multi-target melee attacks, but very little in the way of defense. Reaver probably delivers the most defense of the three due to plate armor, but it'll be hard to get enough strength to actually wear it.

Defender, Paladin and possibly Trickster (Rogue instead of hybrid) have good defense, but lack ways to quickly dispatch groups of monsters.

The classical combo here is thief/druid. You basically invest mainly in dagger mastery and stalker, with some sporadic levels in combat reflexes (to improve a little your attack and defense). Very later on, you can invest some levels in strength of stone (when the dps gained is higher than any investment on dagger or stalker, due to the high cost at high levels).

When increasing stalker, be sure to be able to regain all your mana before it expires, (items with mana regen are very appreciated here).

For example, my 69 lvl thief/druid has:

Stalker: 21
Dagger Mastery: 18
Combat Reflexes: 5
Strength of Stone: 6 (just recently acquired, as the increase in dps were better than using the points in dagger or stalker).

With a dagger of 143 dps as a base, this character gets around 1950 dps. Plus, he has 65% critical hit, but the most important stat is 97% deep wounds, a great aid against critters with thorns, as you can do a much more lethal hit and run tactics.

I know this is late, but passive abilities and trees stack with hybrids. So if you want to have maximized Dagger, you could try a Thief/Thief. Essentially you'll use less points splitting between the two same trees and letting them stack, at the cost of having only one tree type. And its only good for passives.